The semiconductor integrated circuit (IC) industry has experienced exponential growth. Technological advances in IC materials and design have produced generations of ICs where each generation has smaller and more complex circuits than the previous generation. In the course of IC evolution, functional density (i.e., the number of interconnected devices per chip area) has generally increased while geometry size (i.e., the smallest component (or line) that can be created using a fabrication process) has decreased. This scaling down process generally provides benefits by increasing production efficiency and lowering associated costs. Such scaling down has also increased the complexity of processing and manufacturing ICs.
For example, as the need to perform higher resolution lithography processes grows, one lithography technique is extreme ultraviolet lithography (EUVL). EUVL employs scanners using light in the extreme ultraviolet (EUV) region, having a wavelength of about 1-100 nm. Some EUV scanners provide 4× reduction projection printing, similar to some optical scanners, except that the EUV scanners use reflective rather than refractive optics, i.e., mirrors instead of lenses. Additionally, unlike immersion lithography, EUVL scanners operate in vacuum. Conventional techniques using an air gauge or other similar mechanisms may not work for focus monitoring and control in EUV scanners. Alternative techniques for monitoring and maintaining focus of the EUV exposure are, therefore, desirable.